Microbial transformation experiments are being conducted with highly active antitumor compounds from nature. These experiments are being conducted in order to accomplish selective chemical changes in the structures of such compounds under the mild reactive conditions obtainable in fermentations. Microbial transformations of natural antitumor agents will continue to be developed as a general means for: a) preparing quantities of potentially active metabolites; b) providing difficult to synthesize drug metabolites, the availability of which will facilitate drug metabolism studies in animals and man; c) elucidating new and potentially important pathways of metabolism of structurally complicated antitumor compounds; and d) establishing a working basis for further structure activity relationship studies on complex biologically-active compounds from nature. Studies will be continued with the Vinca alkaloids, selected quassinoids and other terpenes.